March 13, 2001 — With the international space station going up, and Russia’s Mir space station coming down, it’s a good time to “get down” about gravity. Understanding weightlessness is important not only for space exploration, but for everybody who gets satellite TV.

A rocket flying into space, or a vehicle orbiting the earth, never escapes gravity. Gravity is always pulling on it. Vehicles that achieve weightlessness are just flying away from the earth at precisely the same rate that gravity is trying to pull them back down.

How gravity works
Let’s start from the top. Gravity is a rate of acceleration. That’s why falling objects fall faster and faster, until they reach terminal velocity, when friction and surface resistance keep them from accelerating any more. Gravity’s rate of acceleration is 32 feet per second per second.

For a NASA rocket to achieve escape velocity — the speed necessary to get far enough away from the mass of the earth so gravity is weak enough that a spacecraft can settle into orbit — it has to be going seven miles per second, or 25,800 mph!

How about a pebble, or a feather? Same speeds apply. The rate of gravity is constant. If you vary the weight of the object, you just have to vary the power necessary to get it going fast enough.

Since it’s the mass of an object that gives it gravitational pull, as you get farther from that mass, gravity weakens. The farther you get from Earth, the weaker gravity gets. For example, a mountain climber who gets to the summit weighs less than he did at base camp, and not just because of all the calories he burned getting there. He’s farther from the center of the earth, so there’s less gravitational effect. Mountain climbing isn’t much of a way to diet, though. Even at the top of Mount Everest, you will only weigh an ounce or two less than you do at sea level.

So spacecraft out in orbit, hundreds of miles away from Earth, aren’t being tugged by gravity as much as you are. But while the gravitational pull may be reduced, it’s not gone completely. It’s still pulling spacecraft down. To stay in orbit, they have to move at just the right speed and angle to pull away from the curve of the earth precisely as fast as gravity is pulling them down. Travel too slowly, and the orbiting craft will drop. Travel too fast, and the spacecraft will outdo the pull from below, flying off into space.

'Escaping' gravity
Astronauts “escape” gravity because they’re moving, inside the spacecraft, at just the right speed and angle so that their flight away from the curve of the earth counteracts the gravity pulling them down.

The higher a spacecraft orbits, the weaker the gravitational pull on it, and the slower it has to go to match that reduced gravitational pull.

Like any other object in low Earth orbit, NASA’s space shuttle travels about 17,500 mph to remain in orbit. The exact speed depends on the altitude, which ranges from 190 miles to 330 miles.

Most of the communications satellites that provide you with satellite TV — or the readings for those Global Positioning System receivers that tell you precisely where on Earth you are — fly at 22,500 miles above the earth, in what’s called geosynchronous orbit. At that height, the speed they need to match gravity is 5,968 mph. At that speed, they are travelling along at the same speed that a spot on the spinning earth below them is moving. In effect, they stay over the same spot all the time, so terrestrial transmitters and receivers always know where to aim, and you can watch your ballgames or movies under the firm effect of gravity holding you down in your couch.

David Ropeik is a longtime science journalist and currently serves as Director of Risk Communication at the Harvard Center for Risk Analysis.